Coatings commonly protect substrates from the effects of exposure to severe environmental conditions such as heat, wear and corrosion. A significant factor in the coating's protection ability relates to the manner in which the coating is applied to the substrate. In many industrial applications, coatings are applied via thermal spraying techniques. Two (2) types of thermal spraying apparatus include HVOF (High Velocity Oxygen Fuel) guns and detonation guns.
In a HVOF gun, a continuous high temperature combustion creates a supersonic high energy flow stream. A coating powder interjected into the continuous high energy flow stream, typically within the barrel of the HVOF gun, forms a coating when applied to a substrate. In contrast, the detonation gun, which operates in a pulsed manner, uses kinetic and thermal energy from the detonation of combustible gases to deposit powdered coating materials onto substrates in a pulsed manner. A combustion chamber receives a certain amount of fuel and oxidant gas. A spark plug ignites the combustible gas mixture to initiate combustion which transforms into detonation. The shock wave formed by this detonation travels at a supersonic speed from the combustion chamber into the barrel where a suitable coating powder is typically injected. The shock wave and further expanding detonation products propel the coating powder out of the barrel and deposit it onto a substrate, thereby forming a coating layer. This process repeats until the substrate obtains a sufficient coating thickness. In some detonation spray systems, between successive ignitions, an inert gas, such as nitrogen, is fed into the combustion chamber to halt combustion and prevent backfire into the fuel and oxygen supply, and to purge the combustion chamber and barrel of combustion detonation products.
The mechanics of detonation are key to the operation of the detonation gun. Detonation produces shock waves that travel at supersonic velocities, as high as 4,000 meters per second (m/s), and elevated temperatures, as high as 3,000 degrees Celsius. Detonation within the detonation gun is controlled by the type and amount of fuel (i.e., natural gas, propane, acetylene, butane, etc.), the fuel and oxygen mixture ratio, the initial pressure of the gases in the combustion chamber, and the geometry of the combustion chamber. Cycled ignition of a portion of the combustible mixture creates combustion which increases the entropy within the combustion chamber and, in turn, propagates ignition of the combustible mixture throughout the combustion chamber. With the correct combination of parameters which result in sufficient local pressure and temperature within a given volume, accumulated combustion energy provides transition to detonation.
At a fixed moment in time the detonation wave front is made up of a system of individual detonation cells. The behavior of detonation at the cell level is an important attribute in the control and operation of a typical detonation gun. The detonation cell is a multidimensional structure, which is formed under influence of both the shock wave front and transverse shock waves. The propagation of the shock wave front, created by detonation, is perpendicular to the inner circumference of the combustion chamber and it is directed from the closed end of the combustion chamber to the open end of the combustion chamber. Transverse shock waves also form at the inner circumference of the combustion chamber and move toward and out the central line of the combustion chamber. Under the current description, a detonation wave constitutes the final case of the multidimensional structure of the detonation front that includes a number of traverse shock waves.
The frontal surface of a detonation cell has a convex shape. Behind the frontal surface is a reaction zone where the chemical reactions take place. At the edge of the cell, transverse shock waves form at substantially right angles to the frontal surface of the detonation cell. The transverse waves have acoustic tails that extend from the aft edges of the transverse waves and define the aft edge of the detonation cell. The transverse waves move from cell to cell and reflect off of each other and off of any limiting structure such as the combustion chamber wall. Once detonation has been initiated, the reaction continues in a fairly stable fashion if subsequent detonation cycles are initiated and maintained under similar conditions as the previous detonations.
The shock wave moves from the closed end of the combustion chamber toward the open end of the combustion chamber and into the barrel. It is of particular importance that the combustion chamber be of sufficient length and sufficient diameter to complete the transition from combustion to detonation before entering the barrel, otherwise, the accumulated energy may dissipate within the barrel. It is also important in the operation of a detonation gun to produce a shock wave and direct it to the barrel as efficiently as possible so that a large amount of the kinetic and thermal energy of the gaseous detonation products goes directly to carrying the powder out of the barrel and onto the substrate. However, reflecting transverse waves colliding with other wave structures can collapse, thus diminishing both the speed of the detonation wave and the transfer of detonation energy as it travels through the combustion chamber. These collisions reduce the amount of the energy available to be transferred to the coating powder which decreases the adherence characteristics between the coating and the substrate and lowers the density of the coating itself.
The size of the detonation cell is another important attribute in the control and operation of a detonation gun. Cell size is a function of the molecular nature of the fuel, the initial pressure within the combustion chamber and the fuel/oxygen ratio. The particular cell size for certain conditions can be determined experimentally. The width of a cell, Sc, is measured along the wave front between successive transverse waves. The length of a cell, Lc, is the perpendicular distance from a line tangent to the wave front measured to the intersection point of the acoustic tails from adjacent transverse waves. The typical ratio of cell width, Sc, to cell length is Sc=0.6Lc for the detonable gases under consideration. The physical parameters of a particular detonation gun, such as the geometry and operating pressures, are determined by the cell size of a particular fuel and oxygen mixture.
In a typical detonation gun the components of the detonable mixture are fed into the combustion chamber and, the coating powder is fed directly into the barrel by inert gases ahead of the detonation wave. A certain gas content system and different gases supplied from a continuous source through a valve arrangement of the gun. For example, the operation of the powder valve is coordinated with the firing of the spark plug so that the powder and carrying gases are in position along the barrel to be properly effected by the detonation wave. Typically the gas control valves are opened by mechanical means such as a cam and tappets or a solenoid which pose reliability problems in that they have rapidly moving pieces. The powder valve is responsible for the transportation of the powders that tend to be abrasive in nature leading to gun life cycle and maintenance concerns. In addition, valves pose safety concerns in that a valve that leaks, sticks open or breaks gives an alternate and potentially harmful path for the detonation products to escape. A further disadvantage of these mechanisms is that they often limit the frequency at which the gun can fire because the valve must be opened far enough and long enough to permit the passage of the proper amount of gas through the valve.
The rate at which a detonation gun deposits the coating powder on the substrate is an important economic parameter in industrial applications. The deposition rate is controlled, and at times limited, by a variety of factors such as the type of fuel, the fuel supply system, the geometries of the combustion chamber and barrel, the powder feeder system, the purging of the system between successive initiations and the frequency with which the combustible gas mixture detonates. Deposition rate is expressed as the ratio between the spray rate and the area sprayed ("spray spot square"). The spray rate is stated in terms of the mass of coating powder utilized per unit time, typically Kg/hr, and typically ranges from 1 to 6 Kg/hr. Spray rate is obviously influenced to great extent by the rate at which the combustible gas mixture detonates. In a typical detonation gun a spark plug is the means to ignite the combustible gas mixture and detonates at the maximum rate of 6 to 10 times per second. The spray spot square is the area coated by a single detonation of the gun and is roughly equal to the area of the barrel and is typically expressed as mm.sup.2. A typical industrial detonation gun has a deposition rate of about 0.001 to 0.02 Kg/mm.sup.2 -hr.
In the typical detonation gun the combustible fuels and oxygen are supplied either into a mixing chamber or directly into the combustion chamber itself through a series of valves. The combustible gases are supplied under pressure of about 1 to 3 MPa from a continuous source to the valve system before being issued into the gun. As discussed previously, a valve system, as employed in a typical detonation gun, raises serious concerns about rate, reliability and safety.
An important characteristic affecting coating quality is the supersonic velocities at which the shock waves travel. The shock wave initiates the acceleration of the coating powders, while the detonation products move the coating powders to produce high density coatings with better adhesive qualities than other spray coating methods. The velocity of the coating powder as it exits the barrel is influenced by, among other things, the type of fuel used and the geometries of the combustion chamber and barrel. Typical detonation wave velocities for detonable gas mixtures are about 1,200 m/sec to about 4,000 m/sec. For example hydrogen-oxygen detonation wave velocities are about 2,830 m/sec and methane-oxygen are about 2,500 m/sec. The maximum achievable velocity in prior art detonation gun configurations is approximately 3,000 m/sec.
Another characteristic effecting coating quality is the temperatures surrounding the operation of a detonation gun which effects the coating density. In order to apply a dense coating, the powder must melt within the barrel of the detonation gun. The higher the adiabatic flame temperature of the combustible gas mixture, the easier it is for the coating powder to melt. Typical adiabatic flame temperatures for detonable gas mixtures of concern range from about 1,900.degree. C. to about 3,200.degree. C., with hydrogen-oxygen about 2,807.degree. C. and methane-oxygen about 2,757.degree. C. The heat imparted to the powders is a function of many parameters including the barrel geometry and the active cooling of the barrel. These temperatures are high enough to melt most substrate materials, however, the discontinuous nature of the detonation within a detonation gun and the quick heat dissipation in the atmosphere between the gun barrel and the substrate prevents the substrate from being adversely affected.
The use of non-combustible gases, inert gases, in the operation of a detonation gun also effects the quality of the coatings produced by reducing the density of the coating as well as adversely effecting the adhesion characteristics between the coating and the substrate. Three common uses of non-combustible gases in detonation gun operations include: 1. purging gases; 2. powder carrier gases; and 3. a control on the detonation process. Purging gases typically are inert gases and are used primarily to purge the combustion chamber between successive firings of the spark plug to arrest the combustion process. This is important in the typical detonation gun because the combustion chamber must be filled between successive firings of the spark plug with new amounts of combustible fuel and oxygen mixture through a series of valves. If combustion continued in the combustion chamber while the valves are opened it is possible that the combustion would continue into the fuel and oxidant gas supply and cause an explosion. One of the problems with using purging gases is that they mix with the combustible gases and lower the overall energy of the detonation. Consequently, the heat and kinetic energy available for transferring to the coating powders is reduced and coating density and adhesion are adversely affected.
Powder carrier gases, frequently compressed air, are typically used to transfer the coating powders from a reservoir to the barrel of the detonation gun in front of the detonation wave. In large quantities, these gases also reduce the kinetic energy available for transfer to the coating powders since they decrease the temperature and velocity of the detonation wave front. The effect on coating quality is evidenced by a lower density coating and poor adhesion to the substrate. Finally, inert gases are also mixed with the detonable gases as a control on the detonation process. These gases are typically used in small amounts to control the temperature, velocity and chemical environment of the detonation products, and the detonation stability.
What is needed in the art is a unique self sustained detonation gun.